This invention relates to a process for the preparation of p-xylene as well as the catalytic composition used therein. More particularly, this invention is directed toward a process for the selective conversion of isobutylene to p-xylene over a supported catalyst impregnated with a non-halogen containing water soluble rhenium compound and an alkali metal hydroxide or stannate and then reduced with hydrogen at elevated temperatures.
Dehydrocoupling reactions, such as the conversion of isobutylene to various aromatics, are of interest as a means of utilizing the isobutylene often produced in large amounts as a by-product of such commercial refining processes as hydrocarbon cracking. It would be especially desirable to selectively convert an isobutylene feed to para-xylene while avoiding the formation of ortho-xylene and meta-xylene isomers. Para-xylene is the more valuable xylene isomer and finds applications as an octane booster in motor fuels or as a precursor to commercially important intermediates such as terepthalic acid. While other aromatics like benzene or toluene that may be formed are comparatively easy to distill off, the separation of the xylene isomers is more difficult, requiring costly recrystallization and filtration which may often prove impractical in large scale operations. With the instant invention, a simple and effective process has been devised to obtain the desired para-xylene product in the absence of the troublesome meta- and/or ortho-isomers.
The prior art discloses several methods for the preparation of aromatics from aliphatic or olefinic hydrocarbon streams with the use of catalysts containing some form of rhenium. For example, U.S. Pat. No. 3,546,138 discloses a catalyst consisting of oxides of antimony and iron promoted with an oxide of rhenium for use in the oxidative dehydrogenation of olefins to diolefins and aromatics. However, it is specifically mentioned that isobutylene should not be in the feed in large amounts. U.S. Pat. No. 3,856,880 discloses the use of rhenium to promote ferrite catalysts for the oxidative dehydrogenation of organic compounds; the conversion of a pentene to a mixture of xylenes is specifically taught. In U.S. Pat. No. 3,374,281, a catalyst consisting of an acidic support with a metal or oxide of a metal such as rhenium as a dehydrogenation component is disclosed for the conversion of paraffinic hydrocarbons to various alkylated aromatics. Similarly, a form of rhenium is employed in the catalyst in the following U.S. patents which disclose cyclization or aromatization reactions: U.S. Pat. Nos. 4,003,957; 3,855,115; 3,541,001 and 3,129,243. It should be noted that the listed patents do not describe the selective conversion of an olefinic stream to a single xylene isomer. The prior art which utilizes a rhenium component often does so in conjunction with some other metal and nowhere discloses further treatment with an alkali metal hydroxide or stannate as herein.
The selective production of p-xylene from isobutylene and/or other olefinic charge stocks is disclosed in the following patents: U.S. Pat. Nos. 4,007,231; 3,830,866; 3,769,237; 3,730,957; 3,644,551 and 3,644,550. However, none of these patents disclose catalysts containing any form of a rhenium component. Serious corrosion problems may also arise in some cases from compounds that are added to the catalyst to promote the dehydrocoupling reaction which yields the p-xylene isomer.
The prior art also discloses catalysts which contain rhenium for the isomerization of xylene isomers to p-xylene. See for example U.S. Pat. Nos. 3,642,925 and 3,557,022 and German Pat. No. 2,211,863. These three patents again teach the presence of additional metals, such as platinum or tin, in the catalytic compositions. Furthermore, an isomerization reaction differs radically from the types of coupling reactions claimed herein.